In the process of manufacturing a semiconductor device, a plasma processing apparatus for applying a process to the surface of a semiconductor substrate by using plasma is used. As a plasma processing apparatus, for example, an apparatus for applying a Chemical Vapor Deposition (CVD) process is known. Among plasma processing apparatuses, a parallel plate type plasma processing apparatus is widely used because it is excellent in process evenness and the apparatus structure is relatively simple.
Such a plasma processing apparatus as described above employs an impedance matching device including variable reactance elements, in order to reduce the electricity of a reflected wave from a plasma load to a high frequency power source. The impedance matching device has, for example, a capacitor, and can change its capacitance by a mechanical movement such as rotating an electrode constituting the variable capacitor by, for example, rotation-driving a motor based on results of measuring the electricity of a progressive wave, the electricity of a reflected wave, etc. In other words, the impedance matching device can obtain a match between the output impedance of a high frequency power source and the input impedance to an electrode of the plasma processing apparatus, by changing element constants (capacitance, inductance, etc.) of the built-in variable reactance elements such as the variable capacitor and a variable inductance, etc.
In a case where a process condition is changed, the impedance of the plasma load is greatly changed. Therefore, it is necessary to largely change also the impedance of the impedance matching device.
In this case, if the impedance matching device tries to obtain a match of the impedances by allowing the variable reactance elements constituting the impedance matching device to be adjusted within a large variable range, the range of fluctuation of the variable elements will be inevitably too large. Therefore, there is a problem that it takes a long time to reach the matched state, and the time required for processing a substrate is elongated.
Further, according to the above-described prior art, the element constants of the variable reactance elements are changed by a mechanical movement such as rotation-driving of a motor. Therefore, there is a problem that a long time, for example, equal to or more than several seconds, is required for obtaining a match of the impedances. Furthermore, even if the impedance matching device tries to minutely adjust the capacitance of the variable capacitor in response to a change in the characteristic of the plasma, it is difficult to accurately adjust the impedances because of the roughness of the mechanical control. As a result, the plasma becomes unstable in some case.